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Chapter 4 Software Development
page 4-5
Figure 4.1.2. Two USB Transactions
All bus transactions begin with a token packet from the host. Devices never initiate data transfers on
their own.
The host controller is in charge of scheduling all traffic on the bus. It schedules the appropriate
transactions for the four data flow types at 1-ms intervals, which the USB documentation calls
frames.
Token packet types include data request (IN), data transfer notification (OUT), and command
transfer notification (SETUP).
4-1-6. Packets
A packet is a continuous bit stream starting with the synchronization pattern and flowing in one
direction.
Table 4.1.3. lists the USB packet types.
PID Type PID Name Transmitter Description
Token OUT Host Data transfer notification
IN Host Data request
SOF Host Start of frame
SETUP Host Data transfer notification for control pipe
Data DATA0 Host/Device Data packet PID even
DATA1 Host/Device Data packet PID odd
Handshake ACK Host/Device Receiver accepts error-free packet
NAK Device Transmitter cannot send data
Receiver cannot accept data
STALL Device Control pipe request not supported
Endpoint halted
Special PRE Host
Preamble enabling downstream bus traffic to low-speed devices
4-1-7. Endpoints
The USB provides separate logical communication flows, called pipes, between the host and a USB
function. The function end of a pipe is called the endpoint. This endpoint must provide buffer space
(FIFO) capable of holding at least one data packet (a.k.a. the maximum payload size).
Endpoints and pipes are characterized, at creation, by the direction (from or to host) and by the data
flow type (control, bulk, interrupt, or isochronous). There are provisions for up to 16 pipes in each
direction, for a total of 32 endpoints per function. The token packet that the host transmits to initiate
a transaction specifies the endpoint buffer with the endpoint address made up of the device address
and the endpoint number.
USB device controller capacity is expressed by the number of endpoints it has and such endpoint
specifications as supported data flow types and endpoint buffer sizes. Examining these endpoint
specifications thus reveals whether the USB device controller is suitable for the intended application.